This invention relates to coatings.
It is known from GB-A-2207089 that powder coatings may be used for bonding two surfaces together by exploiting the melt phase to wet out the surfaces prior to effecting full cure to establish the bond. In normal circumstances, especially when the bond is required between sheets or other large-area surfaces, it is found necessary to employ nip rolling or other pressure-applying means (mechanical or vacuum) to enable a good laminate to be produced. Application of pressure, however, tends to press out the melted powder, and this can be of significant disadvantage where the powder is to provide a coloured or opaque coating to a glass or other transparent or translucent substrate. The applied pressure may result in thinning or pressing out of the melt such that on curing there is no solidity and uniformity of coating-colour or -opacity and the backing to the coating shows through. It is one of the objects of the present invention to provide a method of bonding by which such problems can be avoided.
According to one aspect of the present invention there is provided a method of coating wherein different thermosetting materials are laid down in powder form one upon the other on a substrate, and heat is applied to melt and fuse the powders into respective coatings bonded together on the substrate.
The powders may have different rates of cure from one another, and in these circumstances, the higher cure-rate powder may be laid down as a first layer on the substrate, and the lower cure-rate powder may be laid down as a second layer on the first layer. The lower and higher cure-rate materials may be, fox example, epoxy, polyester, or acrylic materials. Powder coatings can be produced with a variety of cure-rate windows so as to give markedly different rates of cure. The powders are commonly characterised in this regard as xe2x80x98high-bakerxe2x80x99 (xe2x80x98low cure-ratexe2x80x99) or xe2x80x98low-bakexe2x80x99 (xe2x80x98high cure-ratexe2x80x99); the terms xe2x80x98high-bake xe2x80x99 and xe2x80x98low cure-ratexe2x80x99 distinguish from the terms xe2x80x98low-bakexe2x80x99 and xe2x80x98high cure-ratexe2x80x99 in that a xe2x80x98high-bakexe2x80x99 or xe2x80x98low cure-ratexe2x80x99 material takes longer to cure at any given temperature than a xe2x80x98low-bakexe2x80x99 or xe2x80x98high cure-ratexe2x80x99 material.
With the method of the invention it is readily possible to obtain a good laminate without the disadvantage of thinning or pressing out of the melt, and in particular to avoid show-through where colour or opacity is required with a transparent or translucent substrate. In the latter respect, and using materials of different cure-rates, the higher cure-rate material may provide a coating with colour or opacity on a transparent or translucent substrate and the lower cure-rate material may be applied to it while the higher cure-rate material is in its initial un-cured state or at least still not fully cured. When this latter material hardens to form a solid and uniform coating, the lower cure-rate material is still in a state to wet the higher cure-rate coating and the backing surface that is pressed onto it, until curing is complete. Even if the pressure applied to the backing to complete the laminate during curing of the lower cure-rate material, presses out that material to be very thin, the already-cured solid-colour or opaque coating visible through the substrate, remains unaffected.
According to another aspect of the present invention there is provided a method of bonding two surfaces together wherein the bond is effected using two bonding materials that have different cure-rates, the material of lower cure-rate being applied to the material of higher cure-rate while this latter material is in the uncured state, prior to curing of both.